1. Field of the Invention
This invention relates to a bearing housing for a turbocharger incorporating an electric motor. More particularly, this invention relates to a method for joining bearing housing segments for turbocharger incorporating an electric motor.
2. Description of Related Art
A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting an engine's power density without significantly increasing engine weight. Thus, turbochargers permit the use of smaller engines that develop the same amount of power as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which contributes to the highly desirable goal of reduced emissions.
Turbochargers include a turbine housing connected to the engine's exhaust manifold, a compressor housing connected to the engine's intake manifold, and a bearing housing connecting the turbine and compressor housings together. A turbine wheel in the turbine housing is rotatably driven by an inflow of exhaust gas supplied from the exhaust manifold. A shaft is rotatably supported in the bearing housing and couples the turbine wheel to a compressor impeller in the compressor housing so that rotation of the turbine wheel causes rotation of the compressor impeller. The shaft connecting the turbine wheel and the compressor impeller defines an axis of rotation. As the compressor impeller rotates, it compresses ambient air entering the compressor housing, thereby increasing the air mass flow rate, airflow density, and air pressure delivered to the engine's cylinders via the engine's intake manifold.
At low engine speeds, exhaust gas is supplied to the turbine wheel at a lower pressure causing the turbine wheel and compressor impeller to rotate slower, resulting in the air entering the compressor housing being compressed less, which results in a so-called “turbo-lag.” In order to minimize turbo-lag and improve turbocharger efficiency, and therefore engine efficiency, it is known to incorporate an electric motor into the turbocharger. This type of turbocharger is commonly referred to as an electrical turbocompound machine or electrically assisted turbocharger. The electric motor is energized at low engine speeds to impart additional torque to the shaft of the turbocharger, which causes the turbine wheel and compressor impeller to rotate faster, increasing the air mass flow rate delivered to the engine than would otherwise be delivered by a non-electrically assisted turbocharger. The electric motor can also be used as a generator, which converts shaft work, i.e., rotation of the shaft, into electrical power. The electrical power produced by the generator can be used to run auxiliary electrical components or to augment engine power.
One example of an electric motor that is incorporated into the turbocharger is a switched reluctance motor (SRM). The principles of operation of SRMs are simple, well known, and based on reluctance torque. SRMs have a stator with concentrated windings and a rotor with no winding. In a typical electrically assisted turbocharger, the SRM is located in a motor cavity defined within the bearing housing. The rotor is integrated with or mounted on the shaft and is positioned generally between a set of shaft bearings. The stator surrounds the rotor and is secured within the bearing housing. A typical SRM may have six stator poles and four rotor poles, denoted as a “6/4 SRM.” The 6/4 SRM has three phases, each phase consisting of two windings on opposite stator poles. The windings in one phase are simultaneously energized and generate a magnetic flux. The magnetic flux created by the windings follows the path of least magnetic reluctance, meaning the flux will flow through the rotor poles that are closest to the energized stator poles, thereby magnetizing those rotor poles and causing the rotor to align itself with the energized stator poles. Electromagnetic torque is produced by the tendency of the rotor poles to align with the energized stator poles. As the rotor turns, different phases will be sequentially energized to keep the rotor turning. For use as a generator, the phases are energized when the stator poles and rotor poles are separating, rather than when they are approaching.
In order to incorporate the SRM into the turbocharger, it is necessary to assemble the rotor and stator into the bearing housing. While typical bearing housings include turbine and compressor side bores which the shaft is inserted through, such bores are generally not large enough to insert the rotor and stator into the bearing housing. As such, the bearing housing must be modified in order to insert and assemble the rotor and stator into the bearing housing.
It is desirable, therefore, to provide a split bearing housing that separates into two bearing housing segments in order to insert and assemble the rotor and stator into the bearing housing. It is further desirable to provide a method of aligning and maintaining an angular orientation of the bearing housing segments relative to each other.